Phosphatase activity as a parameter for assessment of the rhizodegradation potential of poplar clones: Greenhouse dose-response experiment of phytoremediation of oil contaminated soil

Abstract

During rhizo-degradation, certain microorganisms are capable of breaking down hazardous pollutants such as naphtha and its derivatives or other xenobiotics into non-toxic products. Some plant clones with improved plant-microbe interactions have been selected which should display enhanced ability to secrete exudates which stimulate microbial activity in the substrate where phyto-degradation occurs. Metabolic processes performed by microorganisms of the rhizosphere, and in particular enzymatically catalyzed bio-degradation, form the basis of bioconversion processes for persistent and toxic pollutants into non-toxic or less toxic byproducts. Such bio-conversion is in fact the essence of both spontaneous and controlled bio-remediation processes. Thus, these processes can be monitored by measuring the activity levels of highly specific or less specific enzymes. One universal group of enzymes present in microorganisms involved in bioconversion are phosphomonoester-hydrolases: including alkaline, acid and neutral phosphatase enzymes. Phosphomonoester-hydrolase activity is a good indicator of organic load in natural freshwater, as well as during bioremediation processes that occur in artificial and natural ecosystems. The aim of this study was to investigate the rhizo-degradation potential of different poplar (Popullus spp.) clones in oil contaminated soil, and to measure phosphomonoester-hydrolase activity in rhizosphere microflora as an indicator of rhizo-remediation processes. The effect of crude naphtha on the number of aerobic heterotrophic, facultative oligotrophic and naphtha-oxidizing bacteria, along with the phosphomonoester-hydrolase activity present in rhizosphere microflora, was studied using a greenhouse dose-response experiment. Cuttings from three poplar clones were planted into pots with soil loaded with 6 dosage levels of oil contaminated dry soil ranging from 0-6174 mg/kg of crude naphtha. Six months after planting, both the number of bacteria and phosphatase activity in the rhizosphere zone were increased in low contaminated soil (5%), but significantly decreased with increasing of oil contamination (50%) in a dose-response manner, to a maximum 10-fold reduction in 100% naphtha contaminated soil (vs. 0% contaminated soil controls). No differences were observed between different poplar clones with respect to support of rhizosphere bacterial growth. The relationship between phosphatase activity and the abundance of different groups of bacteria suggest that this phosphatase activity is associated with a specific group of bacteria.